A very useful positional transducer is disclosed in U.S. Pat. No. 3,958,203, issued May 18, 1976 in the name of Victor M. Bernin, which is entitled "Positional Transducer Utilizing Magnetic Elements" and is assigned to the assignee of the present invention. This transducer utilizes an elongated, hollow, cylindrical tube of a magnetically saturable material, a sense line that runs through the tube parallel to its elongated axis and a pair of elongated, generally rectangular-shaped magnets of opposite polarity which have a length that is preferably no greater than the length of the tube and which are positioned adjacent diametrically opposite exterior portions of the tube. As the magnets move over the tube, they provide substantially complete saturation of the tube in the portion of the tube over which they extend, while the remaining portion of the tube remains substantially unsaturated. The transducer thereby gives a linear indication of the position of the magnets with respect to the tube when an electrical AC signal or a pulse signal is supplied either directly, or through magnetic coupling from a drive line, to the sense line that runs through the tube.
The use of a ferrite element that is of a toroidal shape and is positioned between two saturating magnets to produce an output signal on a sense line that runs through the core is disclosed in U.S. Pat. No. 3,638,221, issued in the name of Victor M. Bernin on Jan. 25, 1972, and assigned to the assignee of the present invention. The apparatus of the Bernin patent is a keyboard switch in which the entire toroidal core is completely saturated when the magnets are lowered on a keystem into the vicinity of the cores. In order to accomplish the purposes of the Bernin patent, the core of the switch of that patent is positioned so that its axis is parallel to the wide part of the magnets, which are of a substantially greater dimension than is the thickness of the core.
The transducer of the Bernin U.S. Pat. No. 3,958,203, by contrast, uses a hollow, elongated, cylindrical tube constructed of a material which is magnetically saturable, a sense wire that runs through the tube, and two oppositely poled magnets that move along the outside of the tube in order to provide an accurate linear indication of the position of the magnets with respect to the tube on the sense line. In other words, this transducer does not provide a "1" or a "0" output signal, but instead it may be used to accurately determine the position of the saturating magnets with respect to the tube, since the portion of the elongated tube that lies between the magnets is relatively saturated while the remaining portion is relatively unsaturated.
A number of advantages are thereby realized. Because the tube provides a closed flux path, there is no substantial fringing effect at the ends of the saturating magnets; and, therefore, the portion of the tube that is not between the magnets may remain substantially unsaturated. Since the output signal that is provided on the sense wire through the tube is not dependent upon the magnetic characteristics of the tube, but merely upon the position of the magnets with respect to the tube, a very linear output signal is achieved. In addition, problems that effect magnetic sensors that depend on partial saturation of the sensing element, such as temperature variation and aging variation, are also eliminated. Moreover, the magnetic force that is required to operate this type of transducer is not critical because of reliance on saturation of the tube between the magnets to produce the output signal. This is in direct contrast to sensing devices such as those shown in the McAdam U.S. Pat. No. 2,915,637, where the magnetic saturation of the entire toroidal core is affected by the position of the adjacent magnet; and, thus, the McAdam core is subject to the previously mentioned temperature and aging defects; and the device has a critical magnetic force requirement for the control magnet.
Temperature sensors utilizing a pair of elongated magnetically saturable tubes are disclosed, U.S. Pat. No. 3,950,993, issued Apr. 20, 1976, in the name of Edward F. Sidor, which is entitled, "Temperature Sensors with Improved Operating Characteristics Utilizing Magnetic Elements" , and which is assigned to the assignee of the present invention. A plot of inductance vs. temperature for the two elements, thus, provides curves which intersect at the temperature which is to be sensed. Permanent magnets are positioned adjacent the temperature sensing elements in order to provide a mechanism for adjusting the cross-over temperature point of the magnetic elements.
Magnetic cores, such as toroidal-shaped cores, have been previously used for temperature sensing. The prior art methods of temperature sensing utilized transition characteristics of the magnetic core such as the Curie temperature transition and/or first order transitions such as those described in U.S. Pat. No. 3,534,306, issued on Oct. 13, 1970, in the name of Watrous et al. Prior temperature sensing devices of this type relied on the fact that at a certain temperature a drastic change of the magnetic characteristics of the core would occur. Thus, if a wire were wound around the core to form an inductance element, the inductance of the element would change drastically when the predetermined temperature was reached. This required specific core materials that were specially formulated and carefully controlled in order to provide the desired rapid transition at the exact temperature that was desired. A different specially manufactured magnetic core would then have to be substituted in the sensor in order to sense another temperature.
The sensing device of the Sidor U.S. Pat. No. 3,950,993, by contrast, did not depend upon any rapid change of inductance state of a magnetic core. Instead, the inductance of the magnetic elements varied in a gradual manner until the inductance of both elements is approximately equal at a predetermined temperature which is then sensed by the sensing circuit. The advantage of this approach over the prior art devices is that by changing the inductance of the element by changing the number of windings coupled to it, the cross-over point where the two inductances are equal may be changed so that the temperature sensor may be used over wide range temperatures.
The temperature sensor of Sidor U.S. Pat. No. 3,950,993, is achieved by coupling the two inductively wound elements having different inductance vs. temperature characteristics into a four-arm AC inductance bridge circuit having two terminals that are connected to a conventional null detector. When the inductance vs. temperature characteristics of the two elements cross at a predetermined temperature, the inductances are equal; and the null detector indicates that the desired temperature has been reached. Although two magnetic cores have been connected in series to achieve temperature compensation, as is shown in U.S. Pat. No. 3,824,502; issued on July 16, 1974, to Bardash et al, the utilization of two series connected magnetic elements that have different temperature characteristics for sensing temperatures over a relatively large range of temperatures without a transition change of the magnetic state of the element was not achieved by the device of the Bardash et al patent.
In co-pending U.S. Pat. application, Ser. No. 625,784, filed Oct. 24, 1975 and entitled "Two-Core Magnetic Temperature Sensor" , filed in the name of Edward F. Sidor, and assigned to the assignee of the present invention, a temperature sensing circuit utilizing two magnetic cores is described. That application is a continuation of prior U.S. Pat. application Ser. No. 533,364, filed Dec. 16, 1974, and now abandoned. In this circuit, the two cores were connected into a bridge circuit with two other impedances and were coupled to a sensing circuit, such as a null detector, in order to sense the temperature vs. inductance cross-over point at which the inductance of the two cores became equal. As noted above, one advantage of this type of device was that by changing the inductance of the device, the temperature cross-over point could easily be varied; and the temperature sensor could be used over a wide range of temperatures. The prior application contemplated change of inductance by means of varying the number of windings wound on the cores of the two sensing elements. This approach, however, was time consuming since it required disassembly of the sensing unit.
The temperature sensor of the Sidor application allows for the adjustment of the temperature cross-over point by movement of one or more permanent magnets which are positioned adjacent the magnetic sensing elements, so that by adjustment of the position of the movable magnets, the permeability of the magnetic elements may be adjusted in order to vary the temperature cross-over point without disassembly of the circuit.
A further advantageous feature of the temperature sensor of the Sidor application is that either toroidal-shaped cores or elongated tubular magnetic elements, in which the winding consists of the wire that passes substantially along the axis of the tubular element, may be employed. By making the core elongated and tubular in shape, and by making the length of the permanent magnets so that they are somewhat shorter than the length of the tubular elements, a more precise control is achieved because the amount of saturation of the tubular elements can be closely controlled by positioning of the permanent magnet. This is accomplished by selectively, magnetically saturating a predetermined portion of the elongated magnetic elements.
In addition to employment of a single pair of magnetic elements in a two-arm active bridge circuit, two pairs of magnetic elements may be connected to form a four-arm active bridge circuit which is twice as sensitive as a two-arm active bridge circuit as described in the Sidor U.S. Pat. No. 3,950,993.
The previously described sensors, or transducers, of the Bernin U.S. Pat. No. 3,958,203, and the Sidor U.S. Pat. No. 3,950,993 were extremely valuable for applications that required the sensing of a single condition such as temperature, or pressure or position, etc. However, in applications which require that more than one condition be sensed to satisfy a given control function, a different sensor of these types must be used for each condition to be sensed, thereby increasing the cost and the size of the sensing unit.
It is the object of the present invention to retain the advantages of the sensors, or transducers, of the Bernin and Sidor applications while combining a plurality of sensing functions into a single sensor, thereby providing a sensor of reduced cost and size but of increased utility.